As shown in FIG. 1, a rough boring tool of the prior art is mounted on a boring machine or milling machine for doing a rough machining and is provided with a boring rod 10 which is engageable with a tool rotating seat (not shown in the drawing) and is provided in the vicinity of the tail end thereof with a tool hole 11 in which a boring hole tool 20 is fitted in conjunction with a fastening element 12, such as a screw. The prior art boring tool is devoid of a displacement indicating device as well as a feeding adjustment device and is therefore incapable of a direct on-line adjustment in the midst of operation. It is thus necessary to measure the size of the first test boring prior to a visual feeding adjustment of the boring hole tool 20. Such visual adjustment often results in an inaccurate displacement of the boring hole tool 20 on the boring rod 10. In addition, such visual adjustment must be done repeatedly for several times and is thus rather time-consuming.
As shown in FIGS. 2-4, a prior art boring tool is provided at the tail end of a boring rod 10 thereof with a sliding tool seat 13 which is movably fastened therewith. Located between them are dovetail slots 131 by means of which a micro adjusting element 14, such as a bolt, is disposed. As the microadjusting element 14 is turned, the boring rod 10 and the sliding tool seat 13 are driven to displace radially to attain an advancing quantity P, thereby resulting in a radial displacement of the sliding tool seat 13 on the boring rod 10. In the meantime, the microadjusting element 14 is provided in the outer end thereof with the equally-graduated data 15 for detecting the advancement quantity P of the tool.
Such a structure as described above is defective in design in that the advancement quantity is confined by the length of the microadjusting element 14, thereby resulting in a limited advancement quantity of the machining. The adjustment of the machining advancement quantity must be done in a step-by-step manner, as shown in FIG. 4. In other words, a fastening element 141 of the microadjusting element 14 must be first loosened so as to enable the microadjusting element 14 to be reset at zero. Thereafter, a tool bolt 142 is loosened such that the boring tool 20 is advanced appropriately, and that the tool blot 142 is tightened once again to fasten securely the boring tool 20, and further that the boring tool 20 is displaced by the microadjusting element 14. Such an adjustment of the boring tool 20 as described above must be carried out repeatedly and is thus time-consuming and inefficient. In addition, the engagement of the tool is attained by an excess of elements such that the structural rigidity of the tool is seriously undermined, and that the dovetail slots 131 are vulnerable to crack. Moreover, the microadjusting element 14 is vulnerable to metal fatigue after a prolonged use, thereby resulting in discrepancy between the quantity of a displacement that has taken place actually, and the quantity that is shown by the graduations.
The boring tools disclosed by the U.S. Pat. Nos. 5,396,693; 5,611,651; 5,421,681 and 5,427,480 are suitable for use in finish boring. The microadjustment of the tool advancement is achieved by the thread rotation . The microadjusting elements of these prior art boring tools are similar in construction to that which is described above with reference to FIGS. 2-4. In other words, the microadjusting elements of the prior art are prone to become loosened and are complicated in construction. In addition, such microadjusting elements of the prior art have a low rigidity and are not cost-effective.